Fig. 5: Recording I_NaT and I_NaR in RAPNs with reduced extracellular Na⁺ in frontal slices from 20 days post hatch (dph) and adult male zebra finches at 24 °C.

a Myelin-stained parasagittal section containing nucleus RA (from zebrafinchatlas.org¹²⁸; dorsal is up and anterior to the left). Adapted from Karten et al.¹³³. b Detailed view of the area shown in the dashed red rectangle in (a) depicts myelinated fiber bundles that leave RA, run rostrally, and enter the OM (occipitomesencephalic tract). Adapted from Karten et al.¹³³. c Schematic drawing depicts how generating frontal sections (e.g., dashed line with scissors) transects the axonal fibers closer to the somata of RAPNs (black dots in RA), resulting in shorter initial axonal segments of RAPNs than in parasagittal slices. d Overlay of representative transient (I_NaT) and resurgent (I_NaR) Na⁺ currents from 20 dph (green) and adult (black) finches during a −30 mV test potential (top). The green arrow points to the 20 dph I_NaT peak. I_NaR has been enlarged in the inset. e I_NaR/I_NaT measured at −30 mV in 20 dph and adult finches (N (cells/age) = 3/20 dph and 5/adult finches; two-tailed Student’s t test). Data are presented as individual data points with bars as mean values ± SEM. f Average I–V relationship for I_NaT in 20 dph (green) and adult (black) finches (N (cells/age) = 3/20 dph and 5/adult males). Data are presented as mean values ± SEM. Curve fits for the I–V shown in the red lines. g Curve fits for the current activation shown (red) for averaged I_NaT from 20 dph (green) and adult (black) finches where N is the apparent number of transitions in a Markov chain model (equation shown in Methods), τ is the time constant of activation. Scaled I_NaT from individual neurons used for averaging shown for adult (gray) and 20 dph (light green) finches. Arrows point to current onsets measured as the first point deviating from a line fit to the pre-stimulus baseline current. h Representative I_NaT scaled to the peak from 20 dph and adult; the decay phases have been fitted with single exponentials (red) which overlay the respective traces. Note the small persistent Na⁺ current under the dashed line. i Comparison of the time constant of decay for I_NaT in 20 dph and adult finches (N (cells/age) = 3/20 dph and 5/adult males; two-tailed Student’s t test). Data are presented as individual data points with bars as mean values ± SEM.